Dual channel amplifying circuit



1956 J. J. z. VAN ZELST DUAL CHANNEL AMPLIFYING CIRCUIT 2 Sheets-Sheet 1 Filed April 3, 1952 //vv/v7'0/? Johunnes Jocobus Zoolberg v n Zelg zmr Sheets-Sheet 2 J. J. Z. VAN ZELST DUAL CHANNEL AMPLIFYING CIRCUIT Dec. 25, 1956 Filed April 3, 1952 INVENTOR Iliklill-..

Johannes Jacobus Zoolberg von Zelsf United States Patent O DUAL CHANNEL AMPLIFYJNG CIRCUIT Johannes Jacobus Zaalberg Van Zelst, Eindhoven, Nutherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application April 3, 1952, Serial No. 280,224

Claims priority, application Netherlands April 19, 1951 7 Claims. (Cl. 179-171) of the main amplifier, for example due to one of its tubes becoming defective.

The invention, in such circuits, makes it possible to obtain an amplifying curve having a distortion considerably smaller than that of each amplifier separately. It is characterized in that for the feed-back coeflicients of the negative feedback circuits the following conditions are substantially fulfilled:

in which K represents a constant which obviously differs from 1, and in which the negative feedback coefilcients a a b and b are defined in accordance with Vta and Vtb indicating the negative feedback voltages supplied back to the input circuits of the amplifiers A and B respectively, Ea and Eb indicating thevoltages set up across the load impedance when the spareamplifier B or the main amplifier A respectively is made inoperative (without varying their internal resistances), q and q indicating the fractions in which th e input signal voltage is supplied to the input circluits of.'the amplifiers A andjB respectively, pm and #56 indicating the mean gain factors of the amplifiers A andB respectively, measured as a relation between the'vol tages Ba. and Eb respectively and the input voltage of the amplifier concerned, and No representing the amplification of the combined circuit.

More particularly the negativefeedback circuit of the main amplifier is connected to a pointhaving' a voltage which is preponderantly dependent upon the amplific ation of the main amplifier, whereas the negative feed.-

back circuit of the spare amplifier is connected to a point 7 having a voltage which is substantially equallydepend'ent upon the amplification of the main amplifieras upon that of the spare amplifier, whereby in normal operation of 2,775,657 Patented Dec. 25, 1956 the amplifying circuit both of the amplifiers A and B are operative with the main amplifier A providing a relatively greater part and the spare amplifier B providing a relatively smaller part of the output voltage at said load impedance, and whereby in the event of failure of the main amplifier A, the spare amplifier B will automatically provide the entire amount of output voltage at said load impedance with an amplitude substantially equal to the amplitude of the output voltage obtained in normal operation of the amplifying circuit. In order that the invention may be readily carried into effect, it will now be described in greater detail with reference to the accompanying drawing.

"Fig. 1 shows the principle diagram of a circuit according to the invention.

Figs. 2 to 5 show some simplified embodiments, of I trodes-, and in- Fig. 5 the impedances included in the input circuits are different for the two amplifiers.

Fig; 6 shows a more elaborated modification of th circuit shown in Fig. 2, and

Fig. 7 shows a similar modification of the circuit of Fig. 3.

Fig. 8 shows an embodiment which is based on the circuit shown in Fig. 4.

In Fig. 1, the principle of an amplifier according to the invention is given in a general form. Forthe sake of simplicity, the required sources of supply voltage are omitted in the figures.

The signal voltage Vi to be amplified is supplied by way of a first voltage divider having a division ratio (1 and constituted by impedances 1 and 2 to the input circuit of a main amplifier A and byway of a second voltage divider having a division ratio q and constituted by impedances 3 and 4 to the input circuit of a spare amplifier B. Thus, an amplified voltage Va. is produced across an output impedance 5 of the main amplifier A and an amplified voltage Vb is produced across an output impedance 6 of the spare amplifier B.

The two voltages are supplied by way of impedances 7 and 8 respectively to a load impedance 10, across which an output voltage V0 is produced, which is equal to V0=Eu+Eb (l) in which 'Ea represents the voltage which would be pro duced across the load impedance 10 by the main amplifier A, if the spare amplifier B were inoperative, and Eb represents the voltage which would be produced across the load impedance 10 by 'the spare amplifier B, if the main amplifier A were inoperative. Furthermore, a part Vta of the voltageS sv is supplied as a negative feedback voltage by way 'of' afi 'impedance 11 to the input circuit of the main amplifier A and, in a similar manner, a part Vtb of the voltage V]; is supplied by way of an impedance 12 to the input circuit of the spare amplifier B.

Due to the coupling between the output circuits of the amplifiers A and B by way'of the impedances 7, 8 and 10, the voltages Va and Vb are not equal to the voltages Ea and Eb, but are linearly dependent thereon. 'Thus, linear relations in Ea, and Eb apply for the feedback voltages Va and Vtb, viz.

ta a a+ b b tli a a+ b b n, which (la, ab, ba, ,jl 7b represent negative feedback coefficients-of the'voltages Ba. and Eb which are fed back around the amplifiers A and B, viz. (la is the coeificient of signal Ea which i fed back around amplifier A, as is the coefiicient of signal Eb which is fed back around amplifier A, ha is the coefiicient of signal Ea which is fed back around amplifier B, and bb is the coefiicient of signal Eb which is fed back around amplifier B, the values of which are determined by the impedances shown and the values of which may be immediately determined by making one of the two amplifiers A and B alternately inoperative, for example by switching off the heating current and temporarily adding a resistor to compensate for its internal resistance being varied thereby, and dividing the negative feedback voltages Vta and Vtb supplied to the input impedances 2 and 4 by the voltages Ea and Eb thus produced across the load impedance 10.

For the input voltages 8a and 2b of the main amplifier and the spare amplifier respectively we thus find:

The gain factors Ma and ,u respectively of the amplifiers are defined in accordance with and I-lb= respectively After writing the Equations 3 and 2 in Equation 4, the following relations apply:

By eliminating Ea. and Eb from the said Equations 5, expressions are found for said magnitudes in V1, which after adding and dividing by Vi yield as the amplification From this expression it follows that the amplification N and consequently also the distortion of the whole amplifying circuit is still a function of the gain factors p and ,urespectively of the individual amplifiers A and B.

Assuming that No represents the desired amplification, then at given mean values F and t of the gain factors ,u and ab respectively, the negative feedback coefficients a a h l2 are, according to the invention,

in which K is a constant which materially differs from 1, that is to say is smaller than 0.7 or larger than 1.3.

In this case the Expression 6 changes to a I aO l b I 'bO 1 1 l l a a KM qb qbNo I a Fan g us me in which AN=NNo, of the amplification as a function of the gain factors p and [L It is here assumed that in which A,ua ,ua and mum Lb. For this purpose the left-hand and the right-hand term of the Equation 8 is reduced by 1, so that it follows:

" O MG I- b In this expression q& and qb are either equal to 1 or at least not small with respect to 1, r and ,u being considerably greater than N0, and this so much greater as the extent of backcoupling of each individual amplifier. Consequently, the Expression 10, so long as K appreciably differs from 1 and hence is either smaller than 0.7 or larger than 1.3, approximately changes to AN 0 l u l b o )Maub in which the relative variation in amplification AN W;

is considerably smaller than if K=1.

This may be illustrated by a numerical example. Assuming that Ap =A;L -A[L; q =q :1 and in which, as is common practice, g is at least larger than 10. We thus find in the case that K=1:

but in the case according to the invention in which, for example, K=0:

This makes a difierence of a factor ZA/L which factor is small with respect to 1.

From the fact that the constant K must appreciably differ from 1, it follows that the output circuits and the negative feedback circuits of the amplifiers are required to be chosen diiferent if, for example, the same signal voltages are supplied to the input circuits of the amplifiers and hence the voltage dividers 1, 2 and 3, 4 have the same division ratios qa and qb. In this case the main amplifier A is coupled by way of an impedance 7, which is of the same order of magnitude as the load impedance 10, to the i latter i'r'npedance' 10; the eoupliiig impedance 8 between the pare-amplifiers; and'thedoad impedance 10 and, if desired, -also the oiitp'u't impedance 6 of thespare amplifier B being omitte'd. Thefeedback coupling'by way of the impedances 11 and-1}2 respectively must be forthe main amplifier A about a factor" 2 lower than that for the :spare amplifier B. I

Fig. 2 shows a simplified example of'anamplifier described in the preceding paragraph. The main amplifier A is coupled to load impedance 10 by way of a separatingzimpedance 7, which is, for example, equivalent to load impedanceflO, whereas 'the spare amplifier B is coupled to load impedance 10 without the interposition of any appreciable impedance. Consequently, in order to satisfy :the Conditions 7 for equal amplification both in the: caseithat ,only the :amplifier A is operative and in the case that :only-the spare amplifier .B is operative, ,the back coupling 'ofthe :main amplifier A is. required to be a factor 2 less-than -thatof the spare amplifier v;B. If, for example, the coupling impedances 1 an'd 3 included between the signal source Vrand" the input circuits of the amplifiers A and B are equivalent, .the negative feedback impedance 11 of the mainamplifier A is thus required to have a :value twice that of the negative feedback impedance 12 of the spare amplifier .B'.

However, 'the said-inequality of the output'and negative feedbackcircuits-of the amplifiers A and B- mayralso be obtained by coupling the negative feedback circuit of the .main amplifier A to an output electrode other than the electrode :to which the load impedanceris coupled; In this case ensue, for example, the simplified embodiments' shown in Figs. 3 and 4.

In Fig. =3, .the anode of ,the main amplifying tube A, similarly as that of the spare amplifying tube B, is con nected without the interposition ofiany appreciable impedances to load impedance 10. The negative feedback circuit 11-1 of the main amplifying tube A is, however, coupled to .the screen grid of tube A. The negative-feedback voltage supplied to the control grid of the .spare amplifying tube B is consequently dependent upon both the voltage produced by the main amplifier A and that produced by the spare amplifier B across the load impedance 10, whereas the negative feedback voltagexsupplied to the control grid of the main amplifying tube Aisdependent only upon the screen-grid amplification of the latter tube. Consequently, the negative feedback coeflicient ab and the constant Kare zero.

In a quite similar manner, in the embodiment shown in Fig. 4, the negative feedback voltage'for the m-ainam plifying tube A becomes dependent only upon the amplification of the main amplifier A by the interposition of a cathode impedance 16, whereas that of the spare ainplifying tube B is dependent upon the gain factors'sof .the two amplifying tubes A and B. The negative feedback impedances 16 and 12-3 of course, are still required to be adjusted in accordance with the Conditions 7, whereby the gain factors of the amplifiers A and B are equal when one amplifier is inoperative,

If, on the other hand, the coupling impedances between the corresponding output circuits of the amplifiers A and B and the load impedance 10 are chosen to be equal, for example, by omitting the coupling impedances 7 and 8, the condition that K materially differs from 1 implies that the input circuits and the negative feedback circuits of thea'mplifiers A and B are required to be different. Fig. shows a simplified example of such a case. M

In this example the full input voltage is supplied to the control grid of the main amplifying tube A, whereas the control grid of the spare amplifying tube B has supplied to it by way of the voltage divider 3, 4, for example, only half of the input voltage, that is to say qa=1 and qb l/z. The cathode circuits of the tubes A and 'B furthermore include a common impedance 19, due to which the negative feedback coefficientsab, be, bb acquire equal values whilst an additional, equivalent negative and b0 respectively. are neglected with respect to and respectively-a ,neglection' which in itself is by all means permissible ,the Conditions 7 are fulfilled, whereby the constant K -V2. M

Fig'. ,6 shows a further elaborated embodiment similar to that shown iii'Fi'g. 2. The main amplifier A comprises amplifying tubes 23, 24 and the spare amplifier B comprises tubes 28 and 29. The coupling impedance 7 included between the output impedance 5 of the main amplifier A and the load impedance 10 is of the same order of magnitude as the load impedance 10, whilst the negative feedback of the main amplifier A by wayof the impedances 25, 2'6, 27 is smaller than that of thetspare amplifier B'by way of the'impedances 30, 31. Assuming that V1 represents the voltage at the input circuits of the amplifiers, Va the voltage across the output impedance 5, Vb the voltage across the impedance 6,

the negative feedback factor and rage the amplification of the main amplifier A, the latter measured asa relation between Va and V1 in the absence of negative feedback and with the amplifier B switched off,

the negative feedback factor and rbgb the amplificationof the spare amplifier B, the latter measured as a relation between Vb andVi in the absence of negative feedback and with the amplifierflA switched off, pa. the voltage across the impedance 6 divided by a voltage suppliedto the impedance 5, measured with the amplifiers A and B switched off, and pb the voltage across the impedance 5 divided by a voltage supplied to the impedance 6, also measured with the amplifiers A and B switched off, we find:

from which after elimination of'Va, it follows for the amplification N:

From this follows by differentiation dN am-Magic.mgbpam) ga (Ha -9.1mm) (rb+rbg g.pa) b +y..+gb+gagb a bpum If impedance 7 were left out, then Pa=pb=1, so that when assuming ra=rb=r (equal amplification of the amplifiers A and B) and gs and gb great with respect to 1, we have a a'i' gb N (swab) However, by providing the impedance 7, which is of the same order of magnitude as the load impedance 10, Pa and pr; become smaller than 1, for example between 0.3 and 0.7, whilst as the condition that amplifier A produces across the impedance 10, with the amplifier B switched 01f, a voltage equal to that produced by the amplifier B across the impedance with the amplifier A switched ofi, there applies rb=rapa and hence N a gb( pam) gwfiU-mm) in which, as before, ga and gb are assumed to be great with respect to 1.

Consequently it will be seen that the coeflicients of dga and dgb, which are a measure of the distortion occurring, become considerably smaller in the latter case. If

in the example given it is assumed, for example, that N=rb=100, pa=pb= /2, ra=200, raga=rbgb=l0,000, we find s iL dads) N 3750 g 9 but when leaving out the impedance 7 and N =ra=rb= 100 "'0 is negligible with respect to However, the conditions may be fulfilled exactly by the interposition of an impedance 32 in the anode circuit of the tube 29, which then must be %X the impedance 6 Fig. 7 shows a modification of the embodiment shown in Fig. 3, in which as before the two amplifiers A and B comprise two amplifying tubes 23, 24 and 28, 29 respectively and in which the negative feedback circuit 30, 31 of the spare amplifier B is coupled to the load impedance 10, so that the negative feedback voltage thus produced is dependent upon the amplification of the two amplifiers, whereas the negative feedback circuit 25, 27 of the main amplifier A is coupled to the screen grid of the last tube 24 thereof and hence is dependent only-upon the amplification of the main amplifier A.

In the embodiment shown in Fig. 8, the amplifiers A and B each comprise three tubes 35, 36, 37 and 38, 39, 40 respectively. The input circuits of the two amplifiers are connected to the signal source V1 without the interposition of any appreciable impedances, the anode circuits of the last tubes 37 and 40 respectively supplying in common the load impedance 10 by way of an output transformer 41. The couplings between the cathode circuits of the tubes 37 and 40 respectively and the input circuits, notably the cathode circuits of the first tubes and 38 respectively of the amplifiers A and B are difierent, however, the cathodes of the tubes 37 and 40 being connected together by way of, for example, equivalent impedances 43 and 44 respectively, and the negative feedback voltage supplied to the cathode of tube 38 of spare amplifier B being derived from the junction between the impedances 43 and 44 and thus being dependent upon the amplification of the two amplifiers A and B. The cathodes of the tubes 37 and 40 are furthermore connected by way of, for example, equivalent impedances 45 and 46 respectively to a point of constant potential, the negative feedback voltage supplied to the cathode circuit of tube 35 of the main amplifier A, however, being derived only from a cathode resistance 45 of tube 37 and hence substantially being dependent only upon the amplification of the main amplifier A.

In the embodiments shown, we always reckoned with a positive value for K. However, it is also possible to give K a negative value in which event the negative feedback voltage of the main amplifier A, so far as it originates from the spare amplifier B, inverses in phase. This is ensured, for example, in the circuit shown in Fig. 6 by providing a coupling impedance (not shown) between the cathodes of the tubes 29 and 23.

The impedances which are shown as resistances, may under certain conditions be substituted for by transformers, in which event one impedance, for example impedance 7, might be constituted by the leakage impedance of such a transformer.

If desired, the impedance 6 may, under certain conditions, form part of the load impedance 10.

What I claim is:

1. An amplifying circuit comprising a main amplifier A and a spare amplifier B, each having an input circuit, an output and a negative feedback circuit, a signal source, means connecting the two input circuits to said signal source, a load impedance, and means connecting the two output circuits to said load impedance, characterized in that for the negative feedback coefficients of the negative feedback circuits the following conditions are substantially fulfilled:

in which K represents a constant having a value which differs from 1 by an amount to at least 30% of l, and in which the negative feedback coeflicients aa, ab, ba and bb are defined in accordance with spare amplifier B or the main amplifier A respectively is made temporarily inoperative by switching off the heating current and temporarily adding a resistor to compensate for its internal resistance being varied thereby, qa and qb represent the fractions in which the signal voltage is supplied to the input circuits of the amplifiers A and B respectively, [L and ,lL represent the mean amplification factors of the amplifiers A and B respectively, measured as a relation between the voltages Ea. or Eb respectively and the input voltage of the amplifier concerned, and N0 represents the amplification of the circuit, whereby in normal operation of the amplifying circuit both of the amplifiers A and B are operative with the main amplifier A providing a relatively greater part and the spare amplifier B providing a relatively smaller part of the output voltage at said load impedance, and whereby in the event of failure of the main amplifier A, the spare amplifier B will automatically provide the entire amount of output voltage at said load impedance with an amplitude substantially equal to the amplitude of the output voltage obtained in normal operation of the amplifying circuit.

2. A circuit as claimed in claim 1, characterized in that the negative feedback circuit of the main amplifier is connected to a point in the output circuit of the main amplifier, the voltage of which is preponderantly dependent upon the amplification of the main amplifier, whereas the negative feedback circuit of the spare amplifier is connected to a point in the output circuit of the spare amplifier, the voltage of which in practice is equally dependent upon the amplification of the main amplifier as upon that of the spare amplifier.

3. A circuit as claimed in claim 2, in which said main and spare amplifiers are provided with output electrodes connected respectively to the output circuits of said main and spare amplifiers, said main amplifier being provided with a second output electrode, characterized in that the negative feedback circuit of the main amplifier is coupled to said second output electrode, the negative feedback circuit of the spare amplifier being coupled to both the load impedance and the output electrode of the second amplifier.

4. A circuit as claimed in claim 2, in which the input circuit of the spare amplifier includes a signal voltage divider, so that the signal supplied to the main amplifier is greater than the signal supplied to the spare amplifier, the main amplifier comprising not only a negative feedback circuit which is common to the two amplifiers, but

10 also a negative feedback circuit which is not common with the spare amplifier.

5. A circuit as claimed in claim 2, in which the negative feedback circuit of the main amplifier is coupled to an output electrode of this amplifier, and including a first coupling impedance connected between the main amplifier output circuit and said load impedance and a second coupling impedance connected between the spare amplifier output circuit and said load impedance, said first coupling impedance having a value of the same order of magnitude as that of the load impedance, this value of impedance being relatively high with respect to the value of said second coupling impedance.

6. An amplifying circuit as claimed in claim 5, characterized in that upon failure of both amplifiers the impedances in the output circuits of the two amplifiers have values at which the quotient of the voltage across the output impedance of the spare amplifier and a voltage supplied to the output impedance of the main amplifier, as well as the quotient of the voltage across the output impedance of the main amplifier and a voltage supplied to the output impedance of the spare amplifier is comprised between 0.3 and 0.7.

7. A circuit as claimed in claim 2, in which each of said amplifiers respectively comprises an amplifier tube having an anode and a cathode, said load impedance being connected in common to said anodes, and including two impedances connected in series between said cathodes, the negative feedback circuit of the spare amplifier being connected to the junction of said last-named two impedances, a source of operating current for said main amplifier, and an impedance connected between said source of operating current and the cathode of the main amplifier tube, the negative feedback circuit of the main amplifier being connected to a point on said last-named impedance.

References Cited in the file of this patent UNITED STATES PATENTS 2,020,317 Jacobs Nov. 12, 1935 2,210,028 Doherty Aug. 6, 1940 2,536,651 Merhaut Jan. 2, 1951 2,605,333 Job July 29, 1952 FOREIGN PATENTS 484,287 Great Britain May 3, 1938 559,258 Great Britain Feb. 10, 1944 56,691 Netherlands July 15, 1952 

